Part 1 of this article appeared in the Summer 2006
edition and described the origin of a research project on strobe
effectiveness in large-volume spaces. That article also described the
project tests, test results and several key factors affecting system
design and performance. In this final installment, additional test
results are presented and the factors affecting system performance are
discussed. Finally, guidelines for strobe system design and installation
in large-volume spaces are presented.

The post-test surveys and interviews
after-each test sought information on how participants were first
alerted, whether they were aware of direct or indirect strobe signaling
and if they encountered blind spots. They were also asked to rate the
system's effectiveness (see Part 1).

Results showed that the participants were
capable of being alerted by both direct and indirect signaling. The
original test hypothesis was that, in large-volume spaces, direct
signaling would be the predominant mode of occupant notification.

Indirect Strobe Effects

In Stores #1 and #3, there were many
aisles where strobes were not located directly overhead. Thus, strobe
light had to come over the racks/shelves from adjacent lines of strobes.
In Store #2, strobes were located directly over almost all aisles. This
resulted in greater indirect coverage on the surface of stock. Similarly,
the greater clearance from the top of the stock to the strobes in Store
#3 versus Store #1 permitted greater penetration into aisles that did
not have strobes directly over them. Figure 1 shows a typical warehouse
store with strobe coverage providing both direct and indirect signaling
to the occupants. Figure 2 is the same diagram highlighted to show the
surfaces where one of the strobes provides indirect signaling by
illuminating the surface of the floor and the stock on the racks or
shelves. The highlighted surfaces in Figure 2 show that as the clearance
between the top of the storage and the strobe is decreased, or as
strobe spacing is increased, light penetration to adjacent aisles is
decreased.

Direct Signaling Effects

These types of stores have large volumes
and long viewing paths. In many places, the aisles, racks and shelves
focus the occupants' vision in a way similar to corridors in schools and
offices. In corridors and hallways less than 20 ft (6 m) wide, the
National Fire Alarm code permits the use of lower-intensity strobes at
greater distances since occupants are likely to directly view at least
one appliance as they transit the corridor. Presently, most authorities
require strobe system design-in warehouses and superstores to be based
on NFPA 72 room coverage requirements. They do not permit the use of
corridor rules.

Participants were asked if they were able
to actually see (directly view) one or more strobe lights flashing
without intentionally looking up at the ceiling. The results show that
the system in Store #3 was the most effective at direct signaling to
occupants. Analysis shows that this was probably the result of a greater
clearance between the top of stock/storage and the strobe lights than
in Stores #1 and #2. As a result, occupants could see more of the
ceiling and, hence, more strobes from most vantage points.

All participants reported that they could
see (directly) at least one strobe as they walked through aisles. The
majority of responses indicate that three to six strobes were generally
visible as the participants moved about the space.

Blind Spots

Nevertheless, as participants walked
around the stores, there still were locations where they did not
directly see a strobe or its indirect reflection. At Store #3, the
greater clearance between stock and strobe lights increased the
likelihood of direct signaling.

Nevertheless, this location also had the
greatest number of reported blind spots where a strobe or its effect was
not visible. Even with a larger strobe clearance, the aisle spacing
versus the strobe spacing resulted in a single row of strobes for three
to five aisles.

Participants found that the strobes did not penetrate
sufficiently to provide reliable direct or indirect signaling when they
were more than two or three aisles away. See Figure 2.

In Store #1, the ceiling was an
open-plantype with all structural members and utilities exposed. The
strobes were located below almost all obstructions except air-handling
ductwork. There were several locations noted where ductwork blocked the
strobes.

Discussion

There was general agreement among
participants that strobe lights were an effective means for alerting the
hearing impaired. The survey results show that the experience of the
participants on that day, with a particular system, affected their
opinions. Their opinions on universal effectiveness correlated with
their opinions of the particular systems they just experienced.
Nevertheless, they gave higher effectiveness ratings when generalizing.
Even though they may have seen faults with the system they just
witnessed, they still felt that strobes were an effective method for
alerting.

To better understand the possible causes of the results of this
project, it is helpful to understand the existing performance based
requirements of NFPA 72 and the light-distribution requirements of UL
1971.1 These are addressed in detail in the complete project report.2 However,
in general, while strobe intensity and spacing are important, the test
results indicate that strobe location with respect to aisle spacing and
stock height may be more important in maximizing occupant notification.

The Store #2 system was generally rated
higher because strobes were located over almost all aisles. The Store #3
system showed that good performance does not require strobes over every
aisle. Similarly, the Store #1 test showed that where aisles are moved,
resulting in not having strobes directly overhead, adequate performance
is still possible.

When all three tests are reviewed and compared, several significant points emerge:

Strobe lights are effective for both
indirect and direct viewing even if not located directly over an aisle,
provided there is sufficient penetration to the aisle.

A design with strobe lights over every
aisle is more effective than one where strobes serve several aisles.

In discussions with participants after
completion of the post-test surveys, the potential for blind spots and
marginalized coverage was discussed. All seemed to agree that designers
should take steps to minimize blind spots and to anticipate rack and
aisle changes (where possible). Many felt that blind spots were an
inevitable result given the complex store layouts and the nature of
visible signaling. Some discussion ensued as to whether blind spots
constituted a "failure."

Many, but not all, participants felt that in the
context of total protection, some blind spots or areas with marginal
coverage would not be cause for concern. Several other conditions
combine to protect store occupants for the short time that they might
not see direct or indirect strobe signaling. First, audible signals
would also provide alerting for hearing-able and many hearing impaired
persons. Second, the occupants are alert and mobile. If they are
anywhere near a fire, other senses (smell, sight, touch) will provide
additional cues. If they are not near the fire, they are not yet
threatened and their normal movement will bring them to an area where
they will be alerted by a strobe if they have not already been alerted
audibly or by other occupants' behavior. Third, if they are not near a
fire, the large volume of the space (to absorb smoke and heat) combined
with sprinkler protection separates them from the threat. Once alerted,
code-compliant means of egress provides several safe ways out of the
space. These occupancies differ from others, such as apartments, offices
and healthcare facilities in that there are typically no dead-ends,
they have good visibility across the space when in main aisles and many
locations have more than two ways for an occupant to move.

This project did not attempt to answer
specific scientific questions, such as what threshold level of
illumination over what area was necessary to alert occupants. Instead,
it sought to simply find out if strobe systems could effectively alert
persons in certain large-volume spaces. Visible signaling by strobes is
only one potential method of occupant notification. This project did not
try to determine what methods (audible, voice, text, etc.) or
combinations of methods are the most effective in obtaining the desired
occupant response.

The results show that use of performance-based design
methods for strobe alerting systems is viable in large warehouse and
superstore-type occupancies. Both direct and indirect signaling
contributed to occupant notification, but only where strobes could
penetrate into the aisles. For a more in-depth analysis and discussion,
readers are referred to the complete report.2

The tests identified variables that are
important to the successful design and installation of strobe systems in
these types of spaces. As a direct result, the NFPA 72 Technical
Committee on Notification Appliances has modified and added Annex text
to the 2007 edition to incorporate some of the findings of this project.

Schifiliti, R., Direct Visual Signaling
as a Means for Occupant Notification in Large Spaces Research Project,
Fire Protection Research Foundation, Quincy, MA, 2006.

Editor's Note About This Article

This is a continuing series of articles
that is supported by the National Electrical Manufacturer's Association
(NEMA), Signaling Protection and Communications Section, and is intended
to provide fire alarm industry-related information to members of the
fire protection engineering profession.

About SFPE

SFPE is a global organization representing those practicing in the fields of fire protection engineering and fire safety engineering. SFPE’s mission is to define, develop, and advance the use of engineering best practices; expand the scientific and technical knowledge base; and educate the global fire safety community, in order to reduce fire risk. SFPE members include fire protection engineers, fire safety engineers, fire engineers, and allied professionals, all of whom are working towards the common goal of engineering a fire safe world.